Presently all d.c. electric motors require a commutator to switch current direction as described hereinafter and examples disclosed in U.S. Pat. Nos. 3,305,740 and 3,694,041, incorporated herein by reference.
When motor fields are manufactured to produce a magnetic field in a fixed direction relative to the motor support structure, a commutator is required on the rotating element called the rotor, to switch current in the rotor fields and produce a rotor magnetic field fixed relative to the motor support structure and displaced from the magnetic field direction of the stationary fields in such a way as to produce torque on the rotor.
In the case of permanent magnet motors, the magnetic field flux is created by permanent magnets. If the permanent magnets are fixed relative to the motor structure, the commutator is required on the rotor for exactly the same reasons as described above.
If the permanent magnets are attached to the rotor, commutating mechanisms, usually electronic (brushless), are required to switch the current in the stationary fields to maintain the stationary magnetic fields in a fixed spatial orientation relative to the rotating magnetic fields produced by the permanent magnets on the rotor. The fixed spatial relationship between the rotating and fixed magnetic fields is required to produce torque on the rotor.
The consequence of the need for d.c. motor commutation depends on the type of commutation.
Mechanical commutation using carbon brushes and a segmented conductive surface on the rotor called the commutator results in the following undesirable characteristics:
1. Reduced life and increased maintenance due to brush and commutator wear PA0 2. Radio frequency interference created by commutator arcing PA0 3. Ripple in motor shaft torque due to commutation PA0 4. Heat generation due to eddy currents resulting from magnetic domain rotation. Expensive motor rotor and field laminations are required to minimize eddy currents and their effects. PA0 5. Increased cost of motor due to brushes, commutator and lamination. PA0 1. Reduced life due to the increased number of electronic parts and complexity of the commutation scheme. PA0 2. Ripple in motor shaft torque resulting from commutation. PA0 3. Heat generation due to eddy currents resulting from magnetic domain rotation. Expensive motor rotor and field laminations are required to minimize eddy currents and their effects. PA0 4. Increased cost due to switching sensors commutation electronics and laminations. PA0 1. Stationary field commutatorless d.c. motors, and PA0 2. Rotating field commutatorless d.c. motors.
Electronic (brushless) commutation used in permanent magnet motors results in the following undesirable characteristics: